1. Field of the Invention
The present invention relates to a switching power supply, and more particularly, the present invention relates to a switching power supply that detects voltage for protecting individual functional circuits from experiencing an overcurrent.
2. Description of the Related Art
In a series-connected voltage stabilizer, when the load of a constant-voltage power supply is short-circuited or overloaded, a switching transistor for control or other function can be damaged. Thus, an overcurrent protection function is provided in the switching power supply to supply the power to individual functional circuits.
FIG. 2 is a circuit diagram showing a conventional switching power supply. A switching power supply 50 has been disclosed in Japanese Unexamined Patent Application Publication No. 6-269159, and includes a switching element 51, a diode D51, choke coils L51 and L52, a comparator-detector 52, and a drive circuit 53.
The switching element 51 is switched on or switched off according to the differential voltage between the input voltage Vi and the output voltage Vo. The diode D51 is a flywheel diode constructed to dissipate the energy (the power) accumulated in the choke coil L51 when the switching element 51 is switched on if the switching element 51 is switched off. The choke coil L51 accumulates the pulse voltage applied when the switching element 51 is switched on as energy. The choke coil L52 reduces ripple noise and spike noise. The comparator-detector 52 is inserted between both ends of the choke coil L52, detects the current flowing in the choke coil L52, and controls the drive circuit 53 which sets the ON-OFF ratio of the switching element 51. The drive circuit 53 outputs the control signal at the pulse so that the output voltage applied to the load (not shown in the figure) is consistent with the input voltage, implements the ON-OFF control of the switching element 51, and detects overcurrent.
As described above, the current is detected by making use of a resistance component of the choke coil L52, and overcurrent protection is realized in a configuration in which the comparator-detector 52 is inserted between both ends of the choke coil L52.
However, since the conventional switching power-supply detects the current by making use of the resistance component of the choke coil in order to reduce ripple noise and spike noise and realizes overcurrent protection, problems occur in that two choke coils, i.e., one choke coil provided for accumulating the pulse voltage applied when the switching element is switched on as energy and another choke coil provided for detecting the current are necessary, the manufacturing cost is increased, and the size of the switching power supply is increased.
In order to overcome the problems described above, preferred embodiments of the present invention provide a switching power supply in which the manufacturing cost and the size thereof are greatly reduced while solving the problems described above.
According to a preferred embodiment of the present invention, a switching power supply includes a switching element, a choke coil and a comparator-detector, wherein first and second voltages are detected from both ends of the choke coil to which the pulse voltage from the switching element is applied, the first and second voltages are compared with each other by the comparator-detector, and the operation of the switching element is controlled according to the output of the comparator-detector to provide overcurrent protection, wherein the first voltage detected from one end of the choke coil is partially divided by first and second resistors and inputted to one input end of the comparator-detector and wherein the second voltage detected from the other end of the choke coil is integrated by third and fourth resistors and a capacitor and inputted to the other input end of the comparator-detector.
According to another preferred embodiment of the present invention, a switching power supply includes a choke coil, a switching element and a comparator-detector, wherein first and second voltages are detected from both ends of the choke coil to which the pulse voltage from the switching element is applied, the first and second voltages are compared with each other by the comparator-detector, and the operation of the switching element is controlled according to the output of the comparator-detector to implement overcurrent protection, wherein first and second resistors are connected in series between a terminal to detect the first voltage of the choke coil and the ground, the connection point of the first and second resistors is connected to one input end of the comparator-detector, wherein a third resistor and a parallel circuit including a fourth resistor and a capacitor are connected in series between a terminal to detect the second voltage of the choke coil and the ground, the connection point of the third resistor to the fourth resistor is connected to the other input end of the comparator-detector, and wherein the output end of the comparator-detector is connected to a drive circuit operating the switching element.
According to another preferred embodiment of the present invention, a switching power supply accumulates the pulse voltage applied when a switching transistor is switched on as energy and detects the current can be achieved by a single choke coil because the first and second voltages are detected from both ends of the choke coil, the first voltage is partially divided by the first and second resistors and inputted in one input end of the comparator-detector, and the second voltage is integrated by the third and fourth resistors and the capacitor, and inputted in the other input end of the comparator-detector.